Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/128

Rh 118 METEOROLOGY [TEMPERATURE. north of the place, by which the sun s rays are never obstructed, it exercises little, if any, influence on the observations ; but if one or more hills obstruct the rays of the sun after it has risen above the horizon, such obstruc tion affects the temperature while, and for some time after, the position in which the thermometer is placed is shaded from the sun. Brewster further made the important remark that the mean of observations made at any pair of hours of the same name, such as 8 A.M. and 8 P.M., 9 A.M. and 9 P.M., &c., does not differ much from the mean temperature of the day. The pairs of hours which approximate closest to the true daily mean are 9 A.M. and 9 P.M., 10 A.M. and 10 P.M., 3 A.M. and 3 P.M., and 4 A.M. and 4 P.M. The mean of four hours at equal intervals from each other gives a result still closer to the true mean temperature. In organizing any system of meteorological observation, by which it is intended to develop the climatology of the country, the de termination of the hours of observation is a question of the first importance. If only two observations be made daily the best hours are 9 A.M. and 9 P.M., or 10 A.M. and 10 P.M.; and if there be four observations the best hours are 3 and 9 A.M. and 3 and 9 P.M., or 4 and 10 A.M. and 4 and 10 P.M. If there be three observa tions the best hours are 9 A.M. and 3 and 9 P.M., or 10 A.M. and 4 and -10 P.M.; but in these cases it is essential that the observations of a minimum thermometer be added to the tempera ture observations. These hours are further strongly recommended by the consideration that they are approximately coincident with the diurnal phases of atmospheric pressure, an exact knowledge of which lies at the root of nearly all climatological inquiries. The three equidistant hours which have been adopted in several countries, viz., 6 A.M. and 2 and 10 P.M., are only good as regards the temperature, not as regards atmospheric pressure. With re spect to two daily observations, the hours 8 A.M. and 1 P.M., which have been adopted in some countries, are singularly unsuitable for the furnishing of the observational data required in the de velopment of the climatologies of these countries ; and, what is still more serious in a science where international co-operation is so imperatively demanded, these observations cannot be used with any satisfaction in such deeply important inquiries as the com parative climatologies of Europe. The times of occurrence of the highest, lowest, and mean daily temperatures, and the amount of the daily range of temperature, are in a great degree influenced by the covering or want of covering of the earth s surface on which the air rests. When the ground is covered with vegetation, the whole of the solar heat falls on the vegetable covering ; and, as none falls immediately on the soil, its temperature does not rise so high as happens where there is no vegetable covering to shade the surface from the sun. The temperature of plants exposed to the sun is not so high as that of exposed soil in the vicinity. As regards forests, the four diurnal phases of temperature occur later than in the open country, and the maximum and minimum are less decided ; and, since the maximum temperature of the air in forests falls short of the maximum in the open to a considerably greater extent than the minimum under trees is above the minimum in the open, it follows that the mean temperature of the air in forests is less than that of the open country adjoining. The reason of the difference is that the chilling effects of nocturnal radiation penetrate lower down among the trees than do the heating effects of solar radiation ; and as the soil is not heated directly by the sun its temperature is lower, and consequently that of the air over it is also lower. A cleared space in a forest, sheltered by the sur rounding trees, but open to the sun, has a warmer and moister atmosphere in spring and summer and very much moister in autumn than prevails in the open country adjoining, and has also the diurnal differences of range peculiar to a warmer and moister atmosphere. One of the most important elements of climate is dis closed by the difference between the hour of lowest and the hour of highest mean temperature respectively, or, as it is usually expressed, by the daily range of temperature. We have seen that as regards the sea in the north-west of Scotland the difference is only 3 and in the Atlantic about 30 N. lat. 8, and that probably the diurnal range of temperature of the surface of the sea nowhere amounts to a degree. In the same part of the Atlantic the daily range of the temperature of the air resting on the ocean is 3 0&amp;gt; 2, and on the sea near land 4 4. On advancing on the land, the daily range of temperature rapidly increases, and the rate of increase is greatly augmented when an inland position is arrived at to which any sea breezes that may prevail do not extend. The true daily range of temperature is stated by obser vations made with maximum and minimum thermometers. Generally speaking, the amount of the range increases as the latitude is diminished, and as the distance from the sea is increased, but above all it increases in proportion to the dryness of the climate. The differences of this vital element of climate are strikingly shown in the meteorology of India. In the Report for 1880 the following are the mean daily ranges of March of that year at a few places : at Goa 5 4, Bombay 11 2, Kurrachee 23 5, Jacobabad 37 4, and Pachbudra (lat. 25 55 N., long. 72 18 E.) 41 &quot; 3. In the last case, undoubtedly one of the greatest mean daily ranges of tem perature meteorology has yet recorded, the mean of the days was 103 4 and of the nights 62 - l. As March is altogether within the season of the north-east monsoon, the general drift of the wind over western India, where these are situated, is from the interior towards the sea, subject as regards Bombay and Goa to the influences of the land and the sea breeze. On the other hand, in June, when the south-west monsoon has fairly set in, the following are the mean daily ranges of temperature at the same places: at Goa 5 6, Bombay 8 2, Kurrachee 10, Jacobabad 27 6, and Pachbudra 24 1. These show in a striking manner the powerful influence of the moister atmosphere spread over India by the south-west monsoon, under which the daily range of tempera ture falls at Kurrachee from 23 5 to 10, and in the excessively arid climate of Pachbudra from 41 3 to 24 l. In these dry climates of the basin of the Indus, whilst the rainfall both in March and in June is practically nil, yet the relative humidity of the atmosphere is widely different. Thus the humidities for March and June respectively at 4 P.M., when the temperature is nearly the maximum for the day, are 48 and 77 for Kurrachee, 18 and 30 for Jacobabad, and 11 and 36 for Pachbudra. It is not so much the amount of cloud that determines the degree of fierceness of the sun s heat in these climates as the relative humidity, or the dryness of the air, as pointed out by Strachey in 1866. Thus at Jacobabad less than half the amount of cloud appears in the sky in June as compared with March, but the relative humidities are 30 and 18, and the daily range of temperature 27 6 and 37 4. If we except the dry arid wastes of Persia and Arabia, there is perhaps no other region of the globe where the daily range of temperature approaches that of the valley of the Indus. Thus in the dry climates of such places as Sacramento (California) in summer it amounts only to about 30, at Madrid to 27, and Jerusalem 24. In central districts in the south of England it is about 20; farther north it falls to 15; and in the islands in the north, whose climate is strictly insular in its character, the summer daily range is only 10. In Arctic regions, such as Spitzbergen and Boothia Felix, the range in winter varies from to 1 ; in May, when the sun has reappeared and continues to rise and set, it rises to 14; but in July, when the sun does not set, the range sinks to 10. But maximum and minimum thermometers not only show the mean daily range of temperature, they are also of great utility in giving observations for the determination of mean temperature. The mean temperature may be accepted as the mean of the twenty-four hourly observa tions of the day. If with such a system of observation daily readings of the maximum and minimum thermometer be compared, the value of the latter observations in questions of mean temperature may be arrived at. Double series of observations of this description have been made at many places. The following shows a comparison of the mean of maximum and minimum daily temperatures with means from observations made twenty-four times daily, the former exceeding the latter means in nearly all cases :